Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Battery charging circuits that use a switch-mode architecture typically employ a switcher circuit to charge a battery. The term “battery” may refer to a single cell configuration or a multiple cell stack configuration (e.g., a 2S configuration, which comprises 2 series-connected cells).
In some designs, a battery charging circuit that uses a buck type switch-mode architecture can boost the charger output voltage back to the input port when the input power supply is disconnected from the circuit. The battery charging circuit can stay stuck in this undesirable state because it cannot distinguish that the input power supply (e.g., wall adapter) has been removed. This can cause a battery that is that is being charged by the battery charging circuit to eventually discharge upon removal of the input power supply. This undesirable behavior can also violate industry specifications such as the Universal Serial Bus (USB) specification.
Consider, for example, the conventional switch-mode architecture charging circuit shown in FIG. 6A. When a power supply is connected to the circuit, current can flow from the power supply to the battery, thereby charging the battery; and to the load, thereby providing power to the load.
If the duty cycle of the switching circuit reaches very high levels, for example, due to a combination of input voltage collapse (e.g., the adapter cable impedance can cause a large IR-drop) and a nearly fully charged battery, it is possible to cause the charger to incorrectly operate in boost-converter mode after the input adapter is removed. The resulting current flows may set up as shown in FIG. 6B. The primary cause of this “boost-back” behavior is negative inductor current being supplied from the battery during the ON time of the low-side FET. Once this happens, there will be a voltage on the input of the charger that is proportional to the battery voltage and boost-mode duty cycle, resulting in the battery eventually discharging when the adapter is removed. This occurrence of the negative inductor current cannot be completely prevented due to inherent accuracy limitations of zero-crossing detection circuits.
Solutions to this unintended operation include monitoring the adapter input and turning off the switcher when the adapter input goes below some threshold. This method restricts the useful range of an adapter. Some solutions take this a step further, and use software to periodically check the BMS circuitry for negative current flow.